“The most likely suggestion is that it comes from a monogram made of the first three letters of the Greek name for Jesus.
In Greek, “Jesus” is ΙΗΣΟΥΣ in uppercase letters and Ἰησοῦς in lower. The first three letters (iota, eta, and sigma) form a monogram, or graphic symbol, written as either IHS or IHC in Latin letters.”
—Patricia T. O’Conner and Stewart Kellerman, “The ‘H’ in ‘Jesus H. Christ’.” Grammarphobia. February 2019.